Magnetic materials for energy applications are usually divided into two main groups: hard magnets (often referred to as permanent magnets) and soft magnets.
Hard magnets typically have coercivity values Hc>10-100 kA/m, whereas for soft magnets typically the coercivity is Hc<1 kA/m. In between these groups the semi-hard magnetic materials include all alloys whose coercivity (Hc) is between that of soft magnetic and hard magnetic materials.
Permanent magnets (hereinafter abbreviated as “PM”) are typically used in electrical machines (motors, generators). The most advanced permanent magnets today are based on rare earth (RE) metals. The term “rare earth” is commonly abbreviated as “RE”. RE is one of the elements of the Lanthanide series in the periodic table of elements. Said Lanthanide series comprise the chemical elements Lanthanum (La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd), Promethium (Pm), Samarium (Sm), Europium (Eu), Gadolinium (Gd), Terbium (Tb), Dysprosium (Dy), Holmium (Ho), Erbium (Er), Thulium (Tm), Ytterbium (Yb), Lutetium (Lu).
RE based magnets are especially important, as they allow machine designs with high performance, high energy efficiency, and overall compactness in dimension. Typical rare earth-based permanent magnets materials are intermetallic alloys based on Nd—Fe—B, (Nd—Dy)—Fe—B, and Sm—Co. A range of additional chemical elements can be present in the magnet bodies in order to optimize specific properties and also the ratios of the base elements can vary within one type of magnets.
Sintered, dense rare earth-based permanent magnets materials exhibit the highest magnetic performance, i.e. the highest coercivity Hc and the highest remanence Br. A drawback of the rare earth-based permanent magnets materials resides in that they resides in that the rare earth elements used are that expensive that their share forms an essential portion of the total cost for manufacturing the magnet body. That disadvantage holds particularly true for those magnet bodies containing heavy rare earth elements (hereinafter referred to as HRE elements). HRE elements are Terbium (Tb), Dysprosium (Dy), Holmium (Ho), Erbium (Er), Thulium (Tm), Ytterbium (Yb) and Lutetium (Lu).
The high total cost depends not only on the high raw material costs of the rare earth metals, but also on the very complex processing route. Because of the high reactivity of RE metals with oxygen, all processing steps have to be performed under protective atmospheres in order to avoid detrimental impact on the magnetic properties. In order to achieve a maximum magnetic performance, particles can be oriented by applying a high magnetic field before and during the pressing step. Such manufactured magnets are usually higher in performance compared to non-oriented grades. All known powder metallurgy processing routes for RE-based permanent magnets are limited to the manufacture of very simple geometries, because the shaping is based on simple uniaxial die-pressing, isostatic pressing, or hot deformation in a uniaxial die-pressing step. Already very simple geometrical features, like a slightly curved surface instead of a flat surface, comes with a significant higher price of the magnet, because expensive additional machining steps (grinding of the hard materials) have to be employed. This limitation to simple geometries is a big limitation and drawback for the design of advanced, more energy efficient machines, which would profit form more complex shaped magnets.
Yet another important property of PM materials for electrical machine applications is there maximum operating temperature. RE based PM materials suffer from demagnetization at elevated temperatures. In the Nd—Fe—B system partial substitution of Nd with heavy rare earth elements (typ. 4 to 6 at. % Dy) can extend the operating temperature from typically 100° C. (for normal Nd—Fe—B) to about 150 to 200° C. (for Dy doped Nd—Fe—B). In advanced machine designs with increased power densities this extended operating temperature are commonly desired. However, the improved temperature stability comes with a high cost. Due to the exceptionally high cost of heavy RE metals, the cost of such Dy doped or other heavy RE doped magnets is significantly higher compared to conventional RE based PM.
A further problem of RE based PM materials is their intrinsically high susceptibility for corrosion. To enable long-term application, alloying elements for improved corrosion behavior or protective coatings have to be applied.
One way of overcoming this drawback resides in arranging the expensive HRE elements selectively in those areas of the magnet body where enhanced magnetic characteristics are actually required and indispensable once built in an electric device, while keeping the rest of the magnet essentially free of HRE.
One approach of lowering the overall costs of a magnet by selective provision of RE elements resides in diffusing Dysprosium (Dy) along the grain boundaries of the magnet body. The magnet body consisting of a Neodymium-Iron Boron alloy (NdFeB) is sintered first by common methods known in the art. After forming the magnet body in a first step, the magnet body is covered with a protective layer on its outer surface where heavy RE properties are undesired whereas areas with heavy RE properties are desired are not covered with the protective layer on the periphery, i.e. the outer surface of the magnet body on a second step. In a third step, heavy RE materials are deposited on the surface of the magnet body that is not covered by the protective layer e.g. via vapor containing Dysprosium. The magnet is then annealed at higher temperatures to enable diffusion of heavy RE along the grain boundaries inside the magnet body. Diffused Dy replaces the Nd in NdFeB grains and the expelled Nd atoms form a continuous layer around the newly formed (Nd, Dy)FeB grains. Such layers also magnetically isolate the grain form the neighboring grains. By this procedure inside the magnetic body a first region having different magnetic properties compared to the second region, i.e. the region proximate to the outer surface/periphery of the magnet body. This process ultimately leads to the improvement of the coercivity for more than 50% without changing the remanence compared to a magnet body produced according to the first step only.
A first problem of that method resides in that the second region can only be at the surface of the body. A second problem of that method resides in that only second regions having a thin overall thickness can be realized. As a result, the design freedom of the second region of the magnet is very limited.
Another approach resides in employing an additive manufacturing method. Additive manufacturing is an emerging technology, which allows the manufacture of complex shaped parts in a layer-wise building process directly from CAD design data. This makes it an attractive manufacturing method especially for complex shapes in a very short time period from the design to the final component. For metals, the building of components can be achieved in a powder bed by employing either a laser beam (SLM: Selective Laser Melting) or an electron beam (EBM: Electron Beam Melting). The method received much attention recently. However, at the moment there are only a limited number of materials (in total roughly below 20 different materials) available and known, which can be produced by this method.
A substantial limitation of today's SLM and EBM methods for metals resides in that the chemical alloy composition and the material microstructure cannot be varied and controlled locally (in small volume elements at the microstructural level). Therefore, it is not possible to build 3D designed, multicomponent microstructures during the building process of the 3D component.
Another approach for producing a permanent magnet is disclosed in WO2013/185967A1. The method according to this approach uses a focused energy beam (laser beam or electron beam) for the selective sintering of powders. The main target of the process is to conserve both the original microstructure and the morphology (shape) of the powder particles of the feedstock. This is achieved by selecting in the sintering process a temperature-time combination, which only leads to the formation of sintering necks between powder particles, thus avoiding microstructural changes (e.g. grain growth, recrystallization) within the particles, and avoiding a change of the morphology of the particles. This means, that the method is naturally limited to a early stage of sintering, where only sintering necks are formed. In this early stage of sintering, substantial densification of the powder by volume shrinkage and pore filling does not occur. Therefore, the described method always leads to a high amount of residual porosity in the final microstructure. Typical values are above 30 to 40 vol. % of porosity.
A major disadvantage of this method resides in that undesired changes in the crystal microstructure and morphology of the particles can only be achieved by the cost of a high residual porosity, for example a magnetic porosity of 30 vol. %. In an embodiment of WO2013/185967A1, a further non-metallic material such as glass or a polymer is added at a fraction below 10 wt. % (weight percent) such that the spherical morphology and microstructure of the magnetic particles remains conserved. That method leads to microstructures and properties, which are similar and comparable to polymeric bonded magnets. In addition the method has the disadvantage of generating a material with very low mechanical strength and toughness, due to the high porosity. In addition, as the particles are connected by sintering necks in a three dimensional network, eddy currents cannot be efficiently reduced, because of high conductivities in the sintering necks. Therefore, the porosity does not improve significantly eddy current losses. Compared to conventionally sintered, dense magnets, the energy density (BH)max and the mechanical performance of magnets obtained by WO2013/185967A1 is low. Thus, magnets of WO2013/185967A1 need more volume for the same performance compared with conventionally sintered, dense magnets. This is a substantial drawback for all kinds of applications (especially for electrical machines), where compact designs with high energy densities are preferred.
Polymer bonded RE magnets consist of magnetic particles (based on RE permanent magnets) in a polymer matrix. With polymer bonded RE magnets the limitation of very simple magnet geometries can be partly overcome, as e.g. injection molding or other polymer shaping methods can be applied. However, these magnets have the drawback of substantially lower magnetic performance (lower energy density, lower polarization, lower coercivity), as those magnets contain a high amount of polymer (typically far above 30 vol. %). Furthermore, the mechanical properties (strength, creep), and maximum operation temperature are substantially lower compared to sintered RE permanent magnets.
Soft magnetic materials play a key-role for electrical applications in transformers, motors, and generators. Various material grades in different alloy compositions are available, like polycrystalline (e.g. Fe, Fe—Si, Ni—Fe, Co—Fe base), amorphous (e.g. Fe—B—Si, Fe—Ni—B—Si, Fe—Si—B—P—Nb), and nanocrystalline (e.g. Fe—Cu—Nb—Si—B) materials. Due to their moderate cost, crystalline Fe—Si based electric sheets (with typ. 3% Si) are widely used in both non-oriented and grain-oriented grades. In order to reduce eddy current losses magnetic cores are usually built up of a laminated stack of many thin sheets (typical. sheet thickness 0.3-0.5 mm). Sheets are produced by elaborate hot and cold rolling mill technology combined with heat treatment steps. The sheets are stamped to the desired dimension and electrically isolated by applying a ceramic or polymer layer between the sheets. The laminated stack has to be mechanically clamped or bonded by an adhesive in a useful way. The whole process of building a laminated core from thin sheets is elaborate, time-consuming and costly. In addition, the stamping process or any deformation of the electric sheets degrades the magnetic properties. Therefore, additional annealing treatments have to be performed to partly recover the initial properties by a release of generated internal stresses. It is known, that core losses can be reduced in general by reducing the thickness of the sheets to a minimum of typ. 0.1 mm. However, this has the drawback of additional cost and complexity in the manufacture of a laminated magnetic core. Rapidly solidified amorphous and nanocrystalline SM materials offer the lowest core losses and provide the highest energy efficiency. A main drawback of these materials is their high material and production cost. In order to achieve an amorphous or nanocrystalline state, the molten material is rapidly solidified from the liquid state at very high cooling rates (typically 104-106 K/s). This can be achieved only by casting very thin ribbons (typ. 20-50 μm) on a rotating copper wheel. As a drawback it is elaborate and costly to produce magnetic cores based on this very thin ribbons. Another drawback of amorphous and nanocrystalline soft magnet materials resides in their typically high susceptibility to corrosion. To protect the ribbons from corrosion and to lower eddy current losses, ceramic or polymeric coatings of the individual ribbons have to be applied.
Summing up, a fundamental drawback of today's soft magnet core technology is the elaborate and costly manufacturing process, which is a consequence of the layered sheet material concept. Moreover, only comparatively simple/basic core geometries can be manufactured, which limits the degree of freedom in design of advanced, more energy efficient electrical devices drastically.